Nylon has been manufactured and used commercially for about fifty years. The first nylon fibers were of nylon 66, poly(hexamethylene adipamide), and nylon 66 fiber is still made and used as the main nylon fiber in the USA; large quantities of other nylon fibers, especially of nylon 6 fiber, from caprolactam, are also made and used, especially in some other countries. Nylon fiber is used in textile fabrics, and for other purposes. For textile fabrics, there are essentially two main fiber categories, namely continuous filament yarns and staple fiber, i.e. cut fiber. Large amounts of nylon filaments are used in small bundles of filaments, without cutting, i.e. as continuous filament yarn, e.g. in hosiery, lingerie and many silk-like fabrics based on continuous filament yarns; the present invention is not concerned with these continuous filament yarns, but with nylon staple fiber, and its precursor tow, which is prepared by very different equipment, and which requires entirely different handling considerations because of the large numbers of filaments that are handled. Nylon staple fiber has been made by melt-spinning nylon polymer into filaments, collecting very large numbers of these filaments into a tow, which usually contains many thousands of filaments and is generally of the order of several hundred thousand in total denier, and then subjecting the continuous tow to a drawing operation between a set of feed rolls and a set of draw rolls (operating at a higher speed) to increase the orientation in the filaments, often with an annealing operation to increase the crystallinity, especially if stretch nylon is not desired, and sometimes followed by crimping the filaments, before converting the tow to staple fiber, e.g. in a staple cutter. One of the advantages of staple fibers is that they are readily blended, particularly with natural fibers, such as cotton (often referred to as short staple) and/or with other synthetic fibers, to achieve the advantages derivable from blending, and this blending may occur before the staple cutter, or at another stage, depending on process convenience.
A particularly desirable form of nylon staple fiber has been used for many years for blending with cotton, particularly to improve the durability and economics of the fabrics made from the blends of cotton with nylon, because the nylon staple fibers have a high load-bearing tenacity, as disclosed in Hebeler, U.S. Pat. Nos. 3,044,250, 3,188,790, 3,321,448 and 3,459,845, the disclosures of which are hereby incorporated by reference. As explained by Hebeler, the load-bearing capacity is conveniently measured as the tenacity at 7% elongation (T.sub.7), and the T.sub.7 has long been accepted as a standard measurement, and is easily read on an Instron machine. (All measurements herein are made on single staple fiber, unless otherwise indicated, taking appropriate care with the clamping of the short fiber, and making an average of measurements on at least 10 fibers; in most of the Examples herein at least 3 sets of measurements (each for 10 fibers) were averaged together to provide the data that is recorded). Hebeler's process involved drawing the nylon fibers to the maximum operable draw ratio, and subjecting them to a heat treatment under drawing tension for at least 1 second at the maximum operable temperature. (The draw ratio is the ratio of the higher speed of the draw rolls to the lower speed of the feed rolls). Of the four Hebeler patents, the last, i.e. U.S. Pat. No. 3,459,845, claimed the process, by drawing and heat-treating the filaments under drawing tension at 165.degree. to 200.degree. C. for a length of time which provided 1,000 to 6,000 degree-seconds exposure, the filaments being drawn and heat-treated under dry conditions at substantially the maximum operable draw ratio within the range of about 3 to 5 which can be used without excessive filament breakage, feeding the drawn filaments to the staple cutter without crimping, and cutting the uncrimped filaments into staple fiber. For convenience, I shall refer to this heat-treating using the conventional term "annealing". Hebeler showed in his Table 1 various operating conditions that he used, and in his Table 2 the properties of the filaments produced under his various conditions, measured as indicated by Hebeler (although Hebeler refers to "yarn", it is clear that Hebeler was not referring to spun yarn, but to the continuous filaments from his tows), and in his Table 3 the Lea Product values for spun yarns of nylon, with cotton, or other fibers. Since then, further refinements and improvements have been made so that the commercially-available uncrimped nylon staple fiber has had the following typical properties, tenacity (hereinafter "T") 6.8 grams per denier (gpd), elongation to break (hereinafter "E.sub.B ") 47%, and T.sub.7 2.4-2.5 gpd. These products have been made by a process essentially as described by Hebeler, and illustrated in Hebeler's FIG. 1 (and also shown schematically in FIG. 1 herein, as described more particularly below), at a speed of 110 ypm (yards per minute), this being the optimum practical speed of the draw rolls that deliver the drawn tow in Hebeler's process. (References to speeds in textile processes have generally been to the speed at which the final product is produced, unless otherwise indicated; all speeds herein are given in ypm unless otherwise indicated). It has long been desirable to increase this speed without significant detriment to the properties that are desired, but this has not been possible consistently with the existing process. Indeed, the filaments begin to break at higher speeds, and the number of breaks becomes excessive if the speed is increased significantly beyond about 130 ypm, to the extent that the process as a whole becomes inoperable.
So, a main object of the present invention is to increase the speed of the process without significant loss of properties in the resulting product. This has long been desirable.
It would also be advantageous to be able to improve the desirable properties, e.g. the T.sub.7, as this would give greater flexibility, e.g. in blending, for instance enabling yarns to be prepared with equivalent strength (for the blends) while reducing the amount of nylon. In this regard, it should be explained that, since the time of Hebeler's original disclosures, it has proved possible to improve and select the properties of cotton fibers, e.g. to obtain a T.sub.7 for cotton up to as much as 2.5 gpd, or even more by careful selection.